This is a non-provisional application based upon an earlier-filed provisional application, Ser. No. 60/009,477 filed Jan. 2, 1996. The present invention relates generally to composite materials containing fibers and a carbon matrix, and more particularly to a reduced-temperature, low-cost process for fabricating carbon fiber-carbon matrix composites of high density.
Carbon fiber-carbon matrix (C--C) composites are a class of lightweight, very high-temperature materials that have a variety of niche applications in the aircraft and aerospace industries. Among these applications, rocket nozzles, reentry nosetips and heat shields, and aircraft brake disks are the most prominent.
C--C composites are fabricated in general by a two-step method in which a porous carbonaceous fiber preform is first assembled by a variety of textile processes that either directly yield useful shapes or produce bulk materials that are subsequently shaped. A typical preform consists of ten to eighty plies of woven carbon fibers. The fibers can be woven into fairly simple two-dimensional geometries or more complex three-dimensional braid architectures, depending upon the configuration and desired properties of the finished C--C composite part. Second, the carbon matrix is formed by impregnating a carbon matrix precursor material into the preform by either repeated cycles of liquid impregnation and pyrolysis or continuous processing by simultaneous gas infiltration and pyrolysis.
During the liquid impregnation and pyrolysis method, a carbon matrix precursor material, such as a fluid carbonaceous resin or pitch, is impregnated into the porous carbonaceous fiber preform by injection, soaking, or a similar technique. This impregnated preform is then pyrolyzed by heating it to temperatures sufficient to thermally decompose the precursor material to leave behind only carbon.
In contrast, during the gas infiltration technique, of which chemical vapor infiltration (CVI) is an example, a porous carbonaceous fiber preform is placed into a furnace filled with a heated flowing hydrocarbon gas such as methane, propane, or propylene. This gas serves as the carbon matrix precursor material. As the gas slowly diffuses into the porous carbonaceous fiber preform, the gas decomposes, or pyrolyzes, to form carbon.
Several patents are illustrative of this basic processing technology. For instance, U.S. Pat. No. 3,174,895 to Gibson, et al., dated Mar. 23, 1965, discloses a method of fabricating artificial carbon or graphite bonded cloth laminates that have flexibility, strength, and electrical property advantages over monolithic artificial graphites. Graphite cloth sheets are painted with a carbonaceous binder, stacked, molded, cured together under pressure and baked to form the laminates. U.S. Pat. No. 3,233,014 to Bickerdike, et al., dated Feb. 1, 1966, and U.S. Pat. No. 3,238,054 to Bickerdike, et al., dated Mar. 1, 1966, disclose a method for fabricating fibrous carbon articles by pyrolytic deposition in which a fibrous preform is heated in a stream of gas containing a gaseous carbon compound so that the gas decomposes to deposit carbon within the preform to form the matrix. Alternatively, the carbon matrix can be formed by a process in which a synthetic carbonaceous resin, such as a furfural alcohol, impregnates the preform and the resin is subsequently polymerized and carbonized. U.S. Pat. No. 3,462,289 to Rohl, et al., dated Aug. 19, 1969, teaches a method of producing high density reinforced carbon and graphite bodies whereby a carbon or graphite fiber preform is made into a shape and impregnated in vacuum by a suitable carbonaceous resin, followed by pressure curing and baking. A low-cost method to fabricate C--C composites by the use of colloidal graphite impregnation techniques is taught in a pending U.S patent application based upon an earlier-filed provisional application, Ser. No. 60/008,112, by James E. Sheehan, filed Oct. 29, 1996.
Conventionally, resins such as furfural alcohol and phenol formaldehyde (phenolic), as well as pitches derived from both coal and petroleum are used as carbon precursor materials. The use of alternative precursor materials in processing graphitic articles is well known. For instance, U.S. Pat. No. 935,180 to Williamson, dated Sep. 28, 1909, and U.S. Pat. No. 963,291 to Horton, dated Jul. 5, 1910, teach the use of a solution containing a carbohydrate such as molasses to impregnate porous graphitic articles. U.S. Pat. No. 4,472,460 to Kampe, et al., dated Sep. 18, 1984, teaches the use of a liquid sugar solution to coat carbon black particles, which, upon pyrolysis, form a continuous coating of electrically conductive carbon char for use in gas diffusion electrodes. U.S. Pat. No. 3,026,214 to Boyland, et al., dated Mar. 20, 1962, teaches the use of solutions of purified sugar to impregnate carbon bodies in repeated high-temperature processing cycles. These art-described processes, which utilize sugar or other carbohydrates as carbon precursors, prefer that the sugar or carbohydrate be dissolved in either water or some other appropriate solvent.
Both the gas infiltration and liquid infiltration processing methods for C--C composites require prolonged and repeated processing at temperatures over 1000.degree. C. in the absence of oxygen. Both techniques are slow and costly. Some CVI techniques can take as long as weeks or even months to produce a finished part, although advances in forced-flow technology have reduced this processing time. During some liquid impregnation and pyrolysis processes, the resin shrinks upon pyrolysis, so the entire impregnation and pyrolysis cycle must be repeated in order to properly densify the part. The number of cycles is determined by the density desired in the C--C composite article, which is in turn determined by the performance and cost requirements of the C--C composite article in application. For instance, a typical desired density for C--C brake disks ranges between approximately 1.5 g/cm.sup.3 and 1.8 g/cm.sup.3 while a typical desired density for rocket nozzles or nosetips ranges between approximately 1.7 g/cm.sup.3 to 1.9 g/cm..sup.3
The method disclosed by U.S. Pat. No. 3,026,214 utilizing a sugar solution as a carbon precursor also requires pyrolysis under pressure to prevent frothing and similar high-temperature treatment in a non-oxidizing environment to achieve a high purity product. In addition, the use of sugars dissolved in water or other appropriate solvents for the techniques disclosed in the art is inefficient. These are significant factors that contribute to the high cost and complexity of processing C--C composites.